These days, many motor vehicles incorporate capacitive sensors. In particular, it is known for a capacitive sensor to be integrated into the handle of a (side or trunk) door of a motor vehicle in order to search for the presence of said motor vehicle users hand in the proximity of said handle. Once the hand is detected, an authentication protocol by radio means is, for example, implemented between the motor vehicle and a key of said motor vehicle, carried by the user. In the event of successful authentication by radio means, the door of the motor vehicle is, for example, automatically unlocked, without the user having had to manipulate said key.
Conventionally, searching for the presence of the users hand in the proximity of the handle equipped with an inductive sensor takes place by searching for a variation in the capacitance of the capacitive sensor. What happens is that the presence of a hand in the proximity of the capacitive sensor causes a variation in the capacitance thereof, more particularly an increase in said capacitance of said capacitive sensor.
For example, it is known to measure the duration of a predefined sequence of charges/discharges of the capacitive sensor with a current of predefined constant intensity. For example, the capacitive sensor is charged until the voltage at its terminals reaches a predefined value Vmax, then discharged until the voltage at its terminals reaches a predefined value Vmin, then charged again until the voltage at its terminals reaches the value Vmax, etc.
The sequence of charges/discharges comprises a predefined number of such successive charges/discharges. Since the charges/discharges of said capacitive sensor take place with a current of predefined constant intensity, the duration of the sequence of charges/discharges depends in principle only on the capacitance of said capacitive sensor. Accordingly, by recurrently measuring the duration of said sequence of charges/discharges, it is possible to detect the presence of the users hand in the proximity of the handle when the measured duration of the sequence of charges/discharges increases significantly.
In principle, the greater the number of charges/discharges in the sequence of charges/discharges the more sensitive the capacitive sensor. Accordingly, it is possible, by increasing the number of charges/discharges in the sequence of charges/discharges, to increase the range of the capacitive sensor (distance from which the users hand can be detected) and/or to reduce the probability of a false alarm (probability of detecting the presence of a hand in the absence of a hand in the proximity of said capacitive sensor) for a given range.
The number of charges/discharges in the sequence of charges/discharges is generally predefined so as to ensure that the range of the capacitive sensor is greater than a predefined minimum range and, for this minimum range, a probability of false alarm less than a maximum predefined probability.
However, it is understood that the more the number of charges/discharges in the sequence of charges/discharges increases, the greater the electrical consumption of the capacitive sensor.
For reducing the electrical consumption of the capacitive sensor, it is known to choose the number of charges/discharges in the sequence of charges/discharges so that the predefined minimum range is ensured, but with a probability of false alarm which may be greater than the predefined maximum probability. Searching for variation in capacitance of the capacitive sensor is then broken down into two phases: a phase of searching for variation in capacitance and, when a variation in capacitance has been detected, a phase of verifying said variation in capacitance.
During the search phase, the duration of the sequence of charges/discharges is measured. When a detection criterion is verified, e.g. when the measured duration is much higher than a reference duration of said sequence of charges/discharges in the absence of a hand in the proximity of said capacitive sensor, the verification phase is executed. The verification phase comprises several successive sequences of charges/discharges, closely spaced in time and identical to the sequence of charges/discharges of the search phase, of which the durations are measured. The same detection criterion is evaluated for each of the durations measured, and the detection of variation in capacitance is confirmed when said detection criterion is verified for each of the durations measured during the verification phase. It is understood therefore that the verification phase, due to the fact that it is based on several consecutive evaluations of the detection criterion, enables the probability of false alarm to be significantly reduced.
In order to further reduce the electrical consumption of the capacitive sensor, it is also known to gradually reduce the frequency of the sequences of charges/discharges during the search phase (standby mode of the capacitive sensor), to the detriment, however, of the responsiveness with which the capacitive sensor may detect the presence of a hand.
However, there is still a need to reduce the electrical consumption of the capacitive sensor during the search phase. This is because a capacitive sensor in a motor vehicle handle spends much more time in searching for the presence of a hand than in actually verifying the presence of a hand. In addition, when the motor vehicle is stopped, the electrical consumption of the capacitive sensor is directly taken from the motor vehicle battery.